Examine source code of Time Spent i

{
  "manifest_version": 2,
  "name": "Time Spent",
  "version": "1.0",
  "description": "Show how long you've been on the current website.",
  "icons": {
    "48": "icon.png"
  },
  "permissions": [
    "tabs",
    "storage"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_action": {
    "default_popup": "popup.html",
    "default_icon": "icon.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{aea4944c-6fdf-4b22-a887-e076dc5f880c}"
    }
  }
}